The Born - Haber Cycle is usually used to calculate lattice enthalpies.
For sodium chloride this is #DeltaH# for:
This can also be calculated using a theoretical electrostatic model assuming that the substance is made from discrete ions.
The work done in separating 2 charges #q_1# and #q_2# from a distance #r# to infinity is given by:
Where #r# is the separation.
By extending this idea to 3 dimensions we can calculate a "theoretical" value for the lattice enthalpy. By comparing the value of the experimental Born - Haber values with those obtained from the theoretical model we can get a measure of the ionic character of the bond.
Sodium chloride gives a very good agreement indicating that the ionic model we use is a good one. There is much more discrepancy between the values for silver chloride indicating more covalent character than would be expected if it were #Ag^+Cl^-#.